In a 2 1/'2-quart casserole, combine the onions, celery, garlic, and margarine or butter.
Microwave on high for 1 to 2 minutes, or until the margarine or butter is melted.
Stir in the flour.
Add the stock and cauliflower.
Cover with a lid and microwave on high for 10 to 15 minutes, or until the cauliflower is tender.
Let stand for 10 minutes.
Working in batches, puree in a food processor or blender on medium speed until smooth.
Return to the casserole and stir in the milk and nutmeg.
Microwave on high for 2 to 3 minutes, or until hot.
Sprinkle with the chives.